1. Field of the Invention
The present invention relates to a magnesium diaphragm, a method for manufacturing the diaphragm, and a speaker for high-tone playback using the diaphragm.
2. Description of the Related Art
A polymer-based material (fibers or resin) or metal-based material has hitherto been preferably used as a diaphragm of a speaker for a high-tone playback (hereinafter called a “high-tone playback speaker”). The form of the high-tone playback speaker using the diaphragm encompasses various types, such as a dome-shaped speaker and a semi-dome-shaped speaker.
A resin film material; e.g., polyimide (PI), polyetherimide (PEI), or polycarbonate (PC), is used for a resin-based diaphragm for high-tone playback purpose. A resin-based diaphragm usually has a low acoustic velocity “c” (m/s). Therefore, the diaphragm has a physical property of a low frequency at which split resonance starts. Therefore, the high-tone playback speaker using any of these materials encounters a problem in playing back sound of a high-frequency range.
A material; e.g., aluminum or titanium, is used as a metal-based diaphragm for high-tone playback purpose. The metal-based diaphragm usually has higher rigidity than a resin-based diaphragm and, hence, has a physical property of being able to acquire a threshold frequency (fh) higher than that of the resin-based diaphragm. Therefore, the high-tone playback speaker using the material has an advantage of the ability to play back sound up to a high frequency range with little distortion.
However, the diaphragm using aluminum or titanium has a low internal loss (tan.). Therefore, when an “fh” has arisen in an audible range from 20 Hz to 20 KHz, a peak or a dip that appears in the high frequency range is larger than in the case of the resin-based diaphragm. Hence, the sound involves a high level of distortion.
In addition, the metal-based diaphragm has a high specific gravity and hence suffers a problem of a drop in efficiency of converting an input signal into output sound pressure, which in turn results in a drop in acoustic sensitivity. To solve the problem, there is adopted a method for reducing the thickness of the diaphragm, to thereby enhance acoustic sensitivity. However, according to this method, the diaphragm itself has decreased rigidity and becomes likely to cause undesired resonance. There arises a problem of the sound emitted from the diaphragm involving a lot of distortion.
In order to solve the foregoing problem in the resin-based diaphragm and that in the metal-based diaphragm using aluminum and titanium, attention has been paid to a diaphragm using magnesium as a metal-based diaphragm.
Specifically, there has been proposed a diaphragm for a speaker using, as material, a magnesium sheet or a magnesium alloy sheet (see, e.g., JP-A-2002-369284). According to the document, the diaphragm using the magnesium sheet or the magnesium alloy sheet is manufactured in the following manner. First, a wire or a plate, which is formed from magnesium or a magnesium alloy, is formed into a sheet material by a cross rolling method, and the sheet material is molded by a pneumatic molding method. As a result, there is manufactured a diaphragm made of a magnesium sheet or a magnesium alloy sheet, which has a thickness of 0.02 to 0.04 mm.
However, magnesium is susceptible to oxidation. When the thickness of magnesium has become about 30 μm or less, the hardness of magnesium increases under the influence of an oxide film, thereby raising a problem of deterioration of a characteristic of magnesium; that is, a high internal loss.
If the thickness of magnesium is increased to about 100 μm or more, the weight of the diaphragm will be increased, thereby raising a problem of deterioration of speaker performance and acoustic sensitivity.
When an effective area of the diaphragm using magnesium is increased, “fh” falls within an audible range, thereby raising a problem of sound involving a high level of distortion.